Anatomy of the Spine
In humans the spine is composed of bony structures called vertebrae, separated by intervertebral discs. One of the main functions of the vertebrae is to provide structural support and protection for the spinal cord. Each vertebrae is comprised of a spinous process, a bony prominence behind the spinal cord, which shields the cord's nervous tissue on the back side, two bony protrusions on the sides called transverse processes, and a “body” in front of the spinal cord which provides a structural support for weight-bearing. The average adult has 24 vertebrae, although at birth 33 are present, this is due to fusion during normal development. The vertebrae are divided by anatomical locations with 7 in the neck, also called the cervical vertebrae, 12 in the middle back, called the thoracic vertebrae, 5 in the lower back, called the lumbar vertebrae, and the sacrum, which is actually formed from five fused vertebrae. The tailbone, called the coccyx is made of three fused vertebrae. Of these, the lumbar vertebrae are the largest, in part since they are responsible for carrying the majority of body weight. Due to this, the lumbar area is associated with the highest level of degeneration and is believed causative for a wide variety of pain-inducing syndromes.
Disc Biology
Separating the vertebrae are soft intervertebral discs that, together with the two facet joints, allow for movement of the vertebrae and therefore provide the ability of the spine to move in various directions. The complex of two facet joints posteriorly and the disc anteriorly is referred to as a spinal segment. The intervertebral disc is composed of the annulus fibrosus, the nucleus pulposus, and the cartilage endplate. The nucleus pulposus is comprised of anionic proteoglycans, such as aggracan, that have high affinity for water, and provide a cushioning and shock-absorbing function. The annulus fibrosis encapsulates the nucleus pulposus, and is composed of concentrically organized layers of collagen fibrils (lamellae). The composition of the nucleus pulposus is distinctly different than the annulus fibrosis since the former consists primarily of a jelly-like substance and high collagen type I, whereas the latter is made of a solid, fibrotic-like texture, primarily containing collagen type II. In the adult the cartilage endplate is composed primarily of hyaline cartilage and serves to separate the nucleus pulposus and annulus fibrosus from the adjacent vertebral bone.
Chronic Back Pain
Musculoskeletal disorders of the spine are an extremely common occurrence associated with debilitating back pain, leading to enormous psychosocial and economic ramifications. Lower-back pain is the leading source of disability in people under 45 years of age, and it results in significant economic losses (Frymoyer J. W. 1997. The economics of spinal disorders. In Frymoyer et al., eds. The adult spine: principles and practice. Philadelphia, Pa.: Lippincott-Raven, 143-50, which is incorporated by reference herein in its entirety). 80% of people in the United States will experience back pain at some point in their lifetime (Lively, M. W. 2002. South Med J 95:642-646, which is incorporated by reference herein in its entirety), and it is the second most common reason for symptomatic physician visits (Hart, et al. 1995. Spine 20:11-19, which is incorporated by reference herein in its entirety). Causes of back pain range from injury induced, which presents as a minor problem, accelerating to a chronic disorder, as well as degenerative spine diseases that lead to degenerative spondylolisthesis and spinal stenosis. The vast majority of chronic back pain is associated with degeneration of the intervertebral disc, which can manifest in many different clinical conditions including spinal stenosis and instability, radiculopathy, myelopathy, and disc herniation.
Although the association between disc degeneration and chronic back pain has been established, disc degeneration can also occur without back pain. Disc degeneration can occur as a natural process, in many individuals asymptomatically. The origin of pain has therefore been termed “discogenic” not necessarily because of the disc degenerative process, but in part due to the granulation tissue that invades the disc space and causes inflammation and nociception (Peng, et al. 2006. Spine 31:560-566, which is incorporated by reference herein in its entirety).
There is a prevalent view that the majority of lower back pain that is associated with disc degeneration is caused by nerve root compression (radiculopathic pain), however, magnetic resonance imaging many times does not detect compression of verves, even in patients which have sciatica (Freemont, et al. 1997. Lancet 350:178-181, which is incorporated by reference herein in its entirety). More recent studies suggest that lumbar disc herniation itself is not the major cause of lower back pain, but instead the discogenic pain is caused by anular disruption, such as an annular tear (Schwarzer, et al. 1995. Spine 20:1878-1883; Saifuddin, et al. 1998. Spine 23:453-457; Ito, et al. 1998. Spine 23:1252-1258; discussion 1259-1260; Moneta, et al. 1994. Spine 19:1968-1974, each of which is incorporated by reference herein in its entirety). The fact that the annulus itself is surrounded by various nerve endings allows the possibility that inflammation associated with the disruptions of the annular rings is what triggers pain (Peng, et al. 2005. J Bone Joint Surg Br 87:62-67, which is incorporated by reference herein in its entirety).